Premix burners of the above-mentioned generic type are known from a number of publications, for example EP 704657, EP 780629.
Premix burners of this type are based on the common operating principle of injecting combustion air and a gaseous and/or liquid fuel into a conically designed swirl generator, mixing therein and generating a swirl flow of a fuel/air mixture, wherein the swirl generator provides at least two conical half shells assembled with a mutual overlap for forming tangential inlet slots for fuel and air. Downstream of the swirl generator is arranged the mixing zone for homogeneously mixing fuel and air before ignition occurs. Ignition and combustion of the mixture occur inside the combustion chamber with a premix flame. Due to the discontinuous transition from the burner into the combustion chamber at the burner outlet the swirl flow becomes instable and ultimately breaks down into an annular flow with a central recirculation zone, in the forward region of which the premix flame forms. The spatial position of the premix flame is determined by the aerodynamic behavior of the swirl flow at the outlet of the mixing zone.
The flow from the swirl generator is directed into the mixing zone via a transition piece.
Transition pieces have been disclosed in EP 1714081 or in WO 2006094939. FIG. 2 is a replica of FIG. 7 of EP 1714081. The cone shell segments 4 of the swirl generator 1 are placed with respect to a burner axis A extending centrally through the premix burner. The cone shells 4 delimit a swirl space conically widening in the direction of flow. In each case two shell segments 4, arranged adjacent to one another, enclose an air inlet slot 5, through which an air flow penetrates into the swirl space. Each individual cone shell segment 4 has a fuel supply line 6 for admixing fuel into the incoming air flow passing through the air inlet slots 5. The individual cone shell segments 4 open out with their downstream end on an inside wall 7 of the transition piece 2. Along a line of intersection 8 the individual cone shells 4 are connected to the inner wall 7 of the transition piece 2. This wall 7 may comprise a frustoconical portion tapering conically in downstream direction.
Current transition pieces share the problem that sharp edges have to be included to guide the swirling flow from an angular discharge cross-section to a circular cross-section. In the past these transition pieces have been found to be a major contributor to the risk of flashback due to streaks of low velocity or of early self-ignition by the creation of local recirculation zones. The shape and characteristics of the end region of the swirler are an important parameter to the overall burner robustness.